1. Field of the Invention
The present invention relates in general to a transmission system of a motor vehicle including a transmission with a plurality of gear trains having respective different gear ratios, and more particularly to an apparatus for controlling a shifting operation of such a transmission.
2. Discussion of the Related Art
There is known a vehicle transmission of a parallel 2-axes type having an input shaft adapted to receive power from a power source, an output shaft adapted to receive the power from the input shaft, and a plurality of gear trains which connect the input and output shaft for power transmission therebetween and which have respective different gear ratios. An example of the parallel 2-axes type of vehicle transmission is disclosed in JP-A-62-93541 (published on Apr. 30, 1987). In this automatic transmission of parallel 2-axes type, a first gear train 12, 32 for establishing a low-gear position of the transmission is disposed to connect an input shaft 10 and an output shaft 30 via a first one-way clutch OWC2 and a first clutch C1, while a second gear train 14, 36 for establishing a high-gear position of the transmission is disposed to connect the input and output shafts 10, 30 via a second clutch C2. A shift-up action of the thus constructed transmission from the low-gear position to the high-gear position can be effected by simply engaging the second clutch C2 while the first clutch C1 is held in the engaged state. The high-gear position is smoothly established owing to a slipping action of the one-way clutch OWC2.
In the known vehicle transmission described above wherein the first and second clutches C1, C2 function as frictional coupling devices, these frictional coupling devices must be operated each time the transmission is shifted. Further, the transmission must be controlled by a control apparatus adapted to automatically select one of the gear positions according to an appropriate one of memory-stored shift patterns or maps, and on the basis of a load currently acting on an engine of the motor vehicle and a current running speed of the vehicle, and control a hydraulic circuit for activating the appropriate frictional coupling devices to establish the selected gear position. Therefore, the control apparatus must have a complicated control arrangement for performing various functions associated with the shifting operation of the transmission, and tends to be expensive.
In the known transmission, only one gear train or power transmitting path is provided for establishing each gear position of the transmission. Accordingly, each gear train and the frictional coupling device incorporated therein must have a torque transmission capacity large enough to withstand the maximum output torque of the vehicle engine. In particular, the gear train and its frictional coupling device for establishing the low-gear or 1st-speed position of the transmission must have a considerably large capacity. Consequently, the size and weight of the transmission tend to be undesirably large.